Thin film crystal growth by laser annealing

ABSTRACT

A layer of material is transformed from a first state to a second state by the application of energy from an energy beam. For example, large direction- and location-controlled p-Si grain growth utilizes recrystallization of amorphous silicon from superpositioned laser irradiation. The superpositioned laser irradiation controls cooling and solidification processes that determine the resulting crystal structure. Specifically, a first laser beam of a first pulse duration is used to melt an amorphous silicon (a-Si) film and to create a temperature gradient. After an initial delay, a second laser beam with shorter pulse duration is superpositioned with the first laser beam. When a-Si is irradiated by the second laser beam, the area heated by the first laser beam becomes completely molten. Spontaneous nucleation is initiated in the supercooled liquid-Si when the liquid-Si temperature drops below the nucleation temperature. However, the central part of the liquid pool subjected to continued heating by the first laser beam cools down slowly. Grains nucleated in the periphery of the fully molten spot can therefore grow into the liquid-Si and extend in length until they collide at the center of the first laser beam spot. The first laser beam prolongs the molten Si phase and induces grain growth in a certain direction. The second laser beam triggers nucleation and controls grain location leading to subsequent lateral grain growth.

BACKGROUND OF THE INVENTION

The present invention generally relates to transformation of a materialfrom a first state to a second state through the use of an appliedenergy beam. In one embodiment of the present invention, large grainpolycrystalline silicon (p-Si) is formed from amorphous silicon by thesuperpositioned application of laser beams. Other embodiments of thepresent invention provide a method for manufacturing thin filmtransistors (TFTs) utilizing superpositioned laser annealing. Merely byway of example, the invention may be applied to the manufacture of TFTsfor flat panel displays such as active matrix liquid crystal displays(AMLCDs), field emission displays (FEDs), and organic light emittingdiode (O LED) displays. However, it would be recognized that methods inaccordance with the present invention have a much broader range ofapplicability, including the formation of optical sensors.

Fabrication of high quality TFTs on transparent substrates is importantfor successful application of super-high-definition AMLCD technology.Excimer laser crystallization (ELC) is an efficient technology forobtaining high-performance p-Si TFTs for advanced flat panel displayapplications. In order to improve both the quality and uniformity ofpoly-Si TFT performance, the formation of high quality polycrystallinesilicon films having carefully controlled grain size and location isoften required. Pulsed laser crystallization of amorphous silicon (a-Si)films—usually effected by nanosecond, ultraviolet (UV) excimer laserradiation—has emerged as a promising new fabrication method. Laserannealing has been shown to be superior to other crystallizationtechniques because of its low fabrication cost and high efficiency. Inaddition, it is a low temperature processing technique because duringthe fast heating and cooling cycle the bulk substrate material isessentially unaffected, except within a submicron-thick thermalpenetration zone adjacent to the heated film. This feature has importantpractical consequences, since it allows the use of inexpensive largearea glass substrates, compared with more expensive quartz substratescapable of withstanding high temperature annealing.

ELC can produce grains of hundreds of nanometers in length depending onthe a-Si film thickness and the sample preheating. However, theprocessing window is narrow because large grains can often only beobtained for laser pulse energy densities inducing near-completemelting. In the so-called superlateral growth regime, unmelted siliconparticles in the vicinity of the film/substrate interface are thought toact as seeds for crystal growth in the lateral direction (i.e. parallelto the film surface).

Lateral crystal growth is important for improving electrical properties(e.g. electric field mobility) of the p-Si used by the TFT devices.Since the grains of p-Si are irregularly distributed, grain boundariesmay deteriorate the electrical properties, reducing switching speed andincreasing power consumption.

Therefore, a simple method and apparatus for easily transforming a layerof material from one state to another, for example from amorphous topolycrystalline, is desirable.

SUMMARY OF THE INVENTION

The present invention generally relates to transformation of a materialfrom a first state to a second state, for example from amorphous topolycrystalline, through application of an energy beam. In one specificembodiment, large grain polycrystalline silicon (p-Si) is formed fromamorphous silicon by the superpositioned application of laser beams.Embodiments of the present invention relate to large direction- andlocation-controlled p-Si grain growth utilizing recrystallization fromsuperpositioned laser irradiation. The superpositioned laser irradiationcontrols cooling and solidification processes that determine theresulting crystal structure. Specifically, a first laser beam of a firstpulse duration is used to melt an amorphous silicon (a-Si) film and tocreate a temperature gradient. After an initial delay, a second laserbeam with shorter pulse duration is superpositioned with the first laserbeam. When a-Si is irradiated by the second laser beam, the area heatedby the first laser beam becomes completely molten. Spontaneousnucleation is initiated in the supercooled liquid-Si when the liquid-Sitemperature drops below the nucleation temperature. However, the centralpart of the liquid pool that is subjected to continued heating by thefirst laser beam cools down slowly. Grains nucleated in the periphery ofthe fully molten spot can therefore grow into the liquid-Si and extendin length until they collide at the center of the first laser beam spot.The first laser beam prolongs the molten Si phase and induces graingrowth in a certain direction. The second laser beam triggers nucleationand controls grain location leading to subsequent lateral grain growth.

One embodiment of a method in accordance with the present invention forfabricating a film of material comprises the steps of providing a layerof material, the layer of material being substantially of a first stateand selected from a conductive material, a semiconductive material, or adielectric material. A first energy beam is applied to the layer ofmaterial at a first time and for a first duration. A second energy beamis applied to the layer of material at a second time and for a secondduration, the second time subsequent to the first time and the secondduration expiring on or before the first duration, such that the layerof material is converted from the first state to a second state.

One embodiment of an apparatus for forming a film of material inaccordance with the present invention comprises a first energy sourceemitting a first energy beam and a second energy source emitting asecond energy beam. A first delivery element is configured to deliverthe first beam to a position on an amorphous silicon film for a firstduration; and a second delivery element is configured to deliver thesecond beam to the position after the first beam and for a secondduration superpositioned within the first duration.

These and other embodiments of the present invention, as well as itsadvantages and features, are described in more detail in conjunctionwith the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified perspective view of a method of laserannealing in accordance with one embodiment of the present invention.

FIG. 2 plots intensity versus time for laser beams utilized in oneembodiment of the present invention.

FIG. 3A illustrates the simplified elliptical cross-sectional profile ofthe Ar⁺ laser beam applied to the backside of the sample.

FIG. 3B plots beam intensity versus distance from the beam center alongthe minor axis (x-axis).

FIG. 3C plots beam intensity versus distance from the beam center alongthe major axis (y-axis).

FIGS. 4A-4D show plan views illustrating a proposed simplified mechanismof crystal growth in accordance with an embodiment of the presentinvention.

FIGS. 5A-5C show SEM images of a sample annealed by excimer laserradiation combined with Ar⁺ laser under a first set of conditions inaccordance with an embodiment of the present invention.

FIGS. 6A-6C show SEM images of samples annealed by excimer laserradiation combined with Ar⁺ laser under a second set of conditions inaccordance with an embodiment of the present invention.

FIGS. 7A ad 7B show two- and three-dimensional atomic force microscopeimages respectively, of the boundary between the long grainpolycrystalline silicon and the surrounding microstructure.

FIGS. 8A and 8B show SEM images of polycrystalline silicon formed byapplication of only Ar⁺ laser pulses.

FIG. 9 shows one embodiment of a simplified apparatus for performinglaser annealing in accordance with the present invention.

FIG. 10A shows variation of the RF signal with respect to the TTL signalmeasured by an oscilloscope for the simplified apparatus shown in FIG.9.

FIG. 10B shows the Ar⁺ laser pulse captured by a Si-detector andrecorded by an oscilloscope for the simplified apparatus shown in FIG.9.

FIG. 11A plots the width of the long p-Si grain growth region versusfluence of the excimer laser.

FIG. 11B plots the width of the long p-Si grain growth region versusduration of application of the Ar⁺ laser.

FIG. 12 shows a simplified plan view of one embodiment of a TFTstructure formed by the method in accordance with the present invention.

FIG. 13 shows one embodiment of a simplified flat panel display deviceformed by the method in accordance with the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The present invention generally relates to transformation of a materialfrom one state to another through the use of an applied energy beam. Inparticular, one embodiment of the present invention relates to largedirection- and location-controlled p-Si grain growth utilizingrecrystallization from superpositioned laser irradiation. Thesuperpositioned laser irradiation controls cooling and solidificationprocesses that determine the resulting crystal structure. Specifically,in one embodiment of a method in accordance with the present invention,a first laser beam of a first pulse duration is used to melt anamorphous silicon (a-Si) film and to create a temperature gradient.After an initial delay, a second laser beam with shorter pulse durationis superpositioned with the first laser beam. When a-Si is irradiated bythe second laser beam, the area heated by the first laser beam becomescompletely molten. Spontaneous nucleation is initiated in thesupercooled liquid-Si when the liquid-Si temperature drops below thenucleation temperature. However, the central part of the liquid poolthat is subjected to continued heating by the first laser beam coolsdown slowly. Grains nucleated in the periphery of the fully molten spotcan therefore grow into the liquid-Si and extend in length until theycollide at the center of the first laser beam spot. The first laser beamprolongs the molten Si phase and induces grain growth in a certaindirection. The second laser beam triggers nucleation and controls grainlocation leading to subsequent lateral grain growth.

FIG. 1 shows a perspective view of a laser annealing method inaccordance with one embodiment of the present invention. Sample 100includes an optically transparent substrate 102 bearing amorphoussilicon film 104. First modulated and shaped laser beam 106 ofwavelength 514.5 nm emitted from an Ar⁺ laser is applied to back side102 a of optically transparent substrate 102. Beam 106 is transmittedthrough optically transparent substrate 102 and heats elliptical region104 a of amorphous silicon film 104. While first laser beam 106continues to irradiate amorphous silicon film 104, second laser beam 108of wavelength 248 nm from pulsed KrF excimer laser is superpositioned onamorphous silicon film 104, heating second region 104 b overlappingelliptical region 104 a. Upon cessation of first laser beam 108 andcooling of the sample, long grain polycrystalline silicon crystals growaround the edge to the center of elliptical region 104.

FIG. 2 presents a timing chart showing operation of one embodiment ofthe method in accordance with the present invention. At an initial timeT₀, first laser beam 106 is applied to the back side of the transparentsubstrate and heats the amorphous silicon. Following a delay A of about2 ms, at time T₁ second laser beam 108 is applied to the amorphoussilicon. Second laser beam 106 irradiates the sample for a duration B ofabout 52 ns which expires at time T₂. First laser beam 106 thencontinues to irradiate the sample for a trailing time period C, but alsoexpires at time T₃ such that irradiation by first beam 106 takes placeover a total duration D of 4 ms.

FIGS. 3A-C illustrates the cross-sectional profile of Ar⁺ laser beam 106delivered to back side 102 a of sample 100. Specifically, FIG. 3A showsthe 1/e² irradiance focal beam shape about center 0 of elliptical beamcross-section 119. FIG. 3B plots beam intensity along minor axis 119 a(the x-axis), and FIG. 3C plots beam intensity along minor axis 119 b(the y-axis). The intensity of beam distribution of both FIGS. 3B and 3Cis Gaussian.

A possible mechanism for the growth of long-grained silicon fromamorphous silicon utilizing the method in accordance with the presentinvention is described in connection with FIGS. 4A-4C.

FIG. 4A shows that at time T₀, the Ar⁺ laser beam is applied to thesample and transmitted through the substrate to amorphous silicon film104, causing heating of the amorphous silicon in elliptical region 170.The Gaussian beam intensity profile described above in FIGS. 3B and 3Ccreates a corresponding temperature gradient 105 between edge 170 a ofelliptical region 170 and center 0 of elliptical region 170.

FIG. 4B shows that at time T₁, the excimer laser is applied to sample100. Cross-section 172 of the excimer laser beam is superpositioned uponthe heated elliptical region 170 created by the modulated pulse of Ar⁺laser irradiation. When the amorphous silicon 104 is exposed toradiation from the excimer laser in this manner, silicon in ellipticalregion 170 becomes completely molten.

After a pulse duration of about 100 ns, FIG. 4C shows expiration of theexcimer laser pulse at time T₂. Upon cessation of the excimer laserpulse, peripheral regions 170 c outside of elliptical region 170 coolrapidly. Spontaneous nucleation and grain growth is initiated in thesupercooled liquid in peripheral regions 170 c when the temperaturedrops below the nucleation temperature. However, because of heating byAr⁺ laser pulse having a longer pulse duration, silicon in ellipticalregion 170 cools less rapidly after cessation of the pulsed excimerlaser beam. Polycrystalline silicon grains nucleated in peripheralregions 170 c are therefore permitted to grow into the molten ellipticalregion 170, extending in length until crystals 174 having grainboundaries 175 from opposite edges meet in the center at center 0. Thisis shown in FIG. 4D.

The growth of long grain polycrystalline silicon crystals from amorphoussilicon laser annealed in the manner described is supported byexperimental results. FIGS. 5A-5C show scanning electron micrograph(SEM) images of samples annealed by excimer laser radiation of fluence174 mJ/cm², occurring at approximately the 2 ms midpoint of a 4 msduration of a pulse of Ar⁺ laser radiation having a power of 938 mW.FIG. 5A is a large scale view illustrating three distinct regions of theannealed silicon: outer region 176 a, middle region 176 b, and innerregion 176 c. Middle region 176 c should correspond to an outer wing ofthe Gaussian intensity profile of the modulated Ar⁺ laser beam as shownin FIGS. 3B-C. Inner region 176 c is composed of laterally-grown p-Sigrains of significant length.

FIG. 5B shows a detail of boundary 176 d between outer region 176 a andmiddle region 176 b. Outer region 176 a consists of small grain sizecrystals evidencing partial melting of the amorphous silicon by theexcimer laser pulse. Middle region 176 b consists of microcrystallinesilicon produced by excimer laser energy densities inducing completemelting of the amorphous silicon film.

FIG. 5C shows a detail of boundary 176 e between the laterally grownpolycrystalline silicon grains of inner region 176 c and the surroundingmiddle microcrystalline region of middle region 176 b. Thepolycrystalline silicon grains of inner region 176 c extend to about 12μm. The present invention is not limited to forming grains of thislength, and may be used to form grains of any length preferably greaterthan 10 μm. Ultimately, the length of the polycrystalline silicon grainsis determined by the size and intensity profile of the cross-section ofthe first laser beam.

FIGS. 6A-6C show scanning electron micrograph (SEM) images of samplesannealed under slightly different conditions than shown in prior FIGS.5A-5C. Specifically, an excimer laser beam of fluence 252 mJ/cm² wasapplied at the midpoint of a 4 ms pulse of Ar⁺ laser radiation of power957 mW. FIG. 6A is a large scale view, and FIGS. 6B and 6C are detailedviews, of long crystal grain portion 150 and boundary 104 e between longgrain crystal portion 150, and surrounding microstructure 152.Microstructure 152 lying outside laterally grown grains 150 comprisesmicrocrystalline silicon where the excimer laser pulse has fully meltedthe amorphous silicon sample. The elliptical shape of laterally growngrains 150 reflects suppression of spontaneous nucleation by the heatinginduced by exposure to the Ar⁺ laser beam. This nucleation suppressionentrains laterally grown grains which meet at the center. The length oflaterally grown grains is around 13 μm.

Atomic force microscopy confirms the topography revealed in FIGS. 5A-Cand 6A-C above. FIG. 7A shows two-dimensional topography of theinterface between long grain polycrystalline silicon 150 and thesurrounding microstructure 152. FIG. 7B shows three-dimensionaltopography of the interface between long grain polycrystalline silicon150 and the surrounding microstructure 150.

To further investigate the role of the Ar⁺ laser in the crystallizationprocess, the sample was annealed with only Ar⁺ laser beams of variouspulse lengths. Results are presented in FIGS. 8A and 8B for pulselengths of 5 and 2 ms, respectively. Both samples exhibitmicrocrystalline structure indicative of partially molten amorphoussilicon films. This further indicates that application of the Ar⁺ laserbeam heats up the amorphous silicon sample prior to application of theexcimer laser pulse. Subsequent to application of the excimer laserpulse, the Ar⁺ laser beam sustains molten phase Si. This suppressesspontaneous nucleation in the supercooled liquid Si surrounding the meltafter expiration of the second laser pulse.

FIG. 9 depicts one embodiment of an apparatus for performing laserannealing of amorphous silicon in accordance with the present invention.Laser annealing apparatus 130 includes first continuous wave Ar⁺ laser132 and second pulsed KrF excimer laser 134.

First Ar⁺ laser 132 emits first laser beam 106 at a wavelength of 532nm. First laser beam 106 is delivered by mirror 120 and lens 122 toacousto-optic modulator (AOM) 124, at which first beam 106 is subjectedto temporal modulation. This modulation of first laser beam 106 isnecessary to prevent excessive heating and damage of opticallytransparent substrate 102 during the annealing process. Radio-frequency(RF) generator 126 provides RF signals 127 to AOM 124 when TTL signal133 is received from function generator 135, thereby defining the pulsewidth of diffracted first laser beam 106 a emanating from AOM 124.

Diffracted first laser beam 106 a is delivered to back side 102 a ofoptically transparent substrate 102 by lens 123, beam expander 127,mirror 128, dichoric mirror 129, and cylindrical lens 131. Cylindricallens 131 imparts an elliptical cross-sectional profile to diffusedsecond laser beam 106 a.

KrF excimer laser 134 emits second laser beam 108 at a wavelength of 248nm. Second laser beam 108 is delivered to amorphous silicon film 104through mirror 110, lenses 112, and beam homogenizer 113. Second laserbeam 108 is split at beam splitter 114 to permit energy meter 116 tomeasure the energy of beam 108.

Timing of application of energy from excimer laser 134 and Ar⁺ laser 132is accomplished by function generator 136. Function generator 135 is inelectrical communication with both excimer laser 134 and Ar⁺ laser 132,and also with RF generator 126. Function generator 135 thus synchronizesthe superpositioned application of laser energy to sample 100, as wellas temporal modulation of first laser beam 106.

Controller 180 controls operation of laser annealing apparatus 130 byexecuting instructions stored in memory 182 in computer readable format.Controller 180 is in electrical communication with, and exerts controlover function generator 135. In this manner, controller 180 determinesthe timing of application of laser energy to the sample.

FIG. 10A shows variation in RF signal 127 output from RF generator 126with respect to TTL signal 133 output from function generator 135, asmeasured by an oscilloscope. FIG. 10B shows the resulting 2 ms Ar⁺ laserpulse captured by a Si-detector and recorded by an oscilloscope. Themeasured delay between generation of a pulse generator fire signal byfunction generator 135 and actual irradiation of sample 100 by excimerlaser 134 is about 1.8 μs, with a standard deviation of 0.203 μs. Giventhe 4 ms pulse duration of the first laser beam as shown in FIG. 2, thisdelay between firing signal and irradiation can therefore be considerednegligible.

The method and apparatus for crystallizing amorphous silicon inaccordance with the present invention offers a number of advantages overexisting techniques.

For example, conventional laser annealing of amorphous silicon oftenrelies upon shaping of the laser beam intensity profile via beam masks,interference, or phase shift masks, all of which are difficult,expensive, and time-consuming to create. Other conventional approachesto laser annealing utilize patterned islands, antireflective coatings,and sample preheating at elevated temperatures, which also require timeand effort and can increase defect rates.

By contrast, laser annealing in accordance with one embodiment of thepresent invention utilizes a much simpler, non-intrusive method thatsimply requires synchronized application of a second laser beam ratherthan fabrication of any particular mask structure over the amorphoussilicon.

Another advantage of laser annealing in accordance with one embodimentof the present invention is a departure from cumbersome and inefficientshaping of large area illumination. Conventional laser annealingtechniques suffer from instability in laser energy from pulse-to-pulse,necessitating slow scanning and irradiation over multiple pulses whichcan give rise to variation in the uniformity and size of the crystalsgrown. These limitations reduce processing speed and hinder practicalintegration of conventional laser annealing into a process flow.

By contrast, laser annealing in accordance with one embodiment of thepresent invention is relatively insensitive to typical variation in thecharacter of the applied laser beams. This is shown below by FIGS. 11Aand 11B.

FIG. 11A plots the width of the long polycrystalline silicon graingrowth region versus fluence of the excimer laser, with the excimerlaser pulse occurring at the midpoint of a 4 ms pulse of an Ar⁺ laserhaving a power of 945 mW. FIG. 11A shows relatively small change in themajor and minor axes of the long crystalline region over a broad rangeof excimer laser fluences.

FIG. 11B plots the width of the long polycrystalline silicon graingrowth region versus duration of application of the Ar⁺ laser pulse,with an excimer laser pulse of fluence of 200 mJ/cm² occurring at themidpoint of the pulse of an Ar⁺ laser having a power of 930 mW. FIG. 11Balso shows relatively small change in the size of the long crystallineregion along the major and minor axes over a variety of durations of Ar⁺laser pulses.

Yet a further advantage of the present method is control over both thelength and directionality of crystal growth. Conventional laserannealing techniques do not permit prolonged exposure of the amorphoussilicon to the laser beam, resulting in only a relatively brief periodof heating and consequently formation of polycrystalline siliconexhibiting only a small grain size. However, formation of large-grainedpolycrystalline silicon in accordance with the present invention is dueto the first modulated beam pulse creating a gradient distribution inthe amorphous silicon and that maintains melting. This combinationsuppresses nucleation and enables prolonged periods of crystal growth tooccur, resulting in elongated crystals.

The present method also permits control over the directionality ofgrowth of the crystal grains. As described extensively above, crystalgrowth is initiated at nucleation sites on the periphery of the moltenregion, followed by extension of the crystal grains toward the center ofthe molten region. By controlling the position and/or shape of thesuperpositioned laser beams, and hence the location of the molten regionin relation to a target, directionality of crystal grain growth can becontrolled. Exercise of control over the directionality of crystalgrowth, and hence the orientation of grain boundaries, is also discussedfurther in detail below in conjunction with scanning of the laser beamsand/or physical translation of the sample.

Laser annealing in accordance with one embodiment of the presentinvention is suited for a wide variety of applications.

One application is in the fabrication of a p-Si TFT. FIG. 12 shows aplan view of such a TFT device 1200 comprising source 1202 and drain1204 separated by channel 1206 of length L. Source 1202, drain 1204, andchannel 1206 are formed from a polycrystalline silicon layer 1205 havingelongated grains 1205 a produced by one embodiment of the method inaccordance with the present invention. Gate 1208 overlies channel 1206and is separated from the underlying channel by a gate dielectric.

Upon application of a potential difference between source 1202 and drain1204, and between gate 1208 and source 1202, charge can be conductedacross channel 1206. However, the precise operation of the p-Si TFTdevice is highly dependent upon the electrical conductioncharacteristics of the channel region 1206.

Where the channel region is composed of a plurality of smaller crystals,the grain boundaries between the crystals will impede movement of chargecarriers across the channel, and the p-Si TFT device will operate atslower switching speeds and require larger applied voltages.

However, where the channel region is composed of high quality, longgrain polycrystalline silicon, charges passing between the source anddrain will encounter few, if any, polycrystalline silicon grainboundaries and therefore experience low sheet resistance. The p-Si TFTwill thus operate with rapid switching speeds at low applied voltages,with high reliability and with high uniformity.

Accordingly, it is a goal of the present invention to fabricate a p-SiTFT device such that a longer grain crystalline region fabricated bylaser annealing of amorphous silicon in accordance with the presentinvention extends at least the length of the channel. This is shown inFIG. 12, wherein single, elongated polycrystalline silicon grains 1205 aextend across entire length L of channel 1206.

As shown in FIG. 12, grains 1205 a have an average size of more than 10μm, but the invention is not limited to producing grains having thesedimensions. Longer or smaller gain sizes are possible, and tailoring ofthe grain size and hence TFT channel length could permit p-Si TFTdevices to be utilized in large scale integration (LSI) circuits thatconventionally require the use of MOS or bipolar transistor devices.

As described above, the dimensions of longer grain polycrystallineregion 1206 are largely dictated by the cross-sectional area of thefirst applied laser beam. However, where the width of the applied beamis narrower than the projected channel length, it is still possible tofabricate the TFT device utilizing the present invention.

Specifically, the laser beams could be scanned for a short distanceacross the material layer between pulse events. This would createoverlapping elongated crystalline regions and further extend the lengthof the grains. In an alternative embodiment, the sample itself could bephysically translated between superpositioned irradiation events topermit growth of crystals of greater length. In such an embodiment, thecrystal would grow in a direction opposite the movement of the sample.In yet another alternative approach, the sample and laser beams couldeach be translated at slightly offset speeds to create elongatedpolycrystalline silicon grains.

Utilizing either stationary or translated samples or beams, as shown inFIG. 12 the direction of boundaries 1205 b of polycrystalline silicongrains 1205 a could be limited to some maximum angle E (E≦45°) from axisF between source 1202 and drain 1204.

FIG. 13 shows a flat panel display device incorporating p-Si TFTstructures fabricated utilizing the method in accordance with thepresent invention. Display device 1300 includes array 1302 of discretepixels 1304. Each pixel 1304 is controlled by p-Si TFT 1306 fabricatedutilizing an embodiment of the present method. Each p-Si TFT isindividually addressable along one of row lines 1308 and column lines1310. Row driver structure 1312 is in electrical communication with rowlines 1308. Column driver structure 1314 is in electrical communicationwith column lines 1310.

Display 1300 further includes memory structure 1316, sensor structure1318, and controller structure 1320. Controller structure 1320 is inelectrical communication with row driver 1312 and column driver 1314,and controls the application of voltages to individually addressableTFTs 1306.

The method for forming large-grain polycrystalline silicon films inaccordance with one embodiment of the present invention can be used tofabricate the source, drain, and channel regions of TFT transistors 1306controlling individual pixels 1304. The present method can also beutilized to fabricate electronic circuits 1312, 1314, 1316, 1318, and1320 that are peripheral to pixels 1304 of array 1302.

Although the present invention has been described above in connectionwith specific embodiments, it must be understood that the invention asclaimed should not be limited to these embodiments. Variousmodifications and alterations in the disclosed methods and apparatuseswill be apparent to those skilled in the art without departing from thescope of the present invention.

For example, while the method and apparatus of present invention isdescribed above in connection with applying superimposed laser beams toopposite sides of a film of amorphous silicon, the present invention isnot limited to such an approach. The superpositioned laser beams inaccordance with the present invention could be delivered to the sameside of a sample, and the method and apparatus would remain within thescope of the present invention.

In addition, while the embodiments described above show formation ofpolycrystalline silicon from amorphous silicon, the present invention isnot limited to annealing this type of amorphous material. Application ofsuperpositioned laser beams could be utilized to transform a variety ofmaterials from one state to another. Materials eligible fortransformation in accordance with the present invention include but arenot limited to semiconductor materials, dielectric materials, andconductive materials. Specific examples of materials that could betransformed from one state to another include silicon-germanium,indium-antimony, and silicon nitride and silicon oxide films.

Moreover, while the embodiments described above utilize laser beams fromdifferent sources (Ar⁺ and KrF excimer lasers) this is not required bythe present invention. A method or apparatus for laser annealing couldutilize a single laser source having its beam split and the respectiveparts applied to the sample at different times and for differentdurations, and the method or apparatus would fall within the scope ofthe present invention.

In addition, while the embodiments described above depict application ofenergy from the second laser beam at the midpoint of the duration ofexposure of the first laser beam, the present invention is not limitedto this timing sequence. Application of the first laser beam to thesample could be timed to expire at the same time as application of thesecond laser beam, and the method and apparatus would remain within thescope of the present invention. In such an embodiment, maintenance ofthe temperature gradient and suppression of nucleation leading toelongated crystal growth would be provided by the slower cooling of thehigher temperature silicon previously exposed to energy from both thefirst laser beam and the second laser beam.

And while the embodiments described above utilize pulse durations on theorder of milliseconds for the first laser pulse and on the order ofnanoseconds for the second laser pulse, this is also not required by thepresent invention. A method could utilize first and second pulsedurations of any length, so long as the pulses are superpositioned andsome delay period exists between commencement of the first laser pulseand the second laser pulse. Preferably however, the first pulse durationis at least 100 nanoseconds and the second pulse duration is less than100 nanoseconds.

Furthermore, the present invention is not limited to annealing amorphoussilicon utilizing laser energy. Energy from sources other than laserscould be applied to melt and recrystallize amorphous material in themanner just described. Such alternative energy sources include but arenot limited to electron beams and ion beams. Application of energy fromsuch sources would be timed to melt and then recrystallize the samplematerial as described extensively above.

In addition, while the above description illustrates application of afirst pulse from a CW laser beam having a Gaussian cross-sectionalintensity distribution, this is also not required by the presentinvention. Laser beams having other cross-sectional profiles, such astop hat-like or another beam shape, could also be utilized to fabricatepolycrystalline silicon in accordance with the present invention.

In addition, the intensity of the modulated CW laser beam could bevariable in time. The rising and falling time of the first beamintensity may be controlled with a precision on the order ofnanoseconds. The cooling rate of the liquid silicon, the solidificationrate of the liquid silicon, and the duration of melting determine thegrowth of polycrystalline silicon grains. Therefore, the variation inlaser intensity over time is an effective parameter to control thepolycrystalline silicon structures.

Furthermore, the shape of the focused CW laser beam could be spatiallymodified to promote crystal growth. Thus instead of the cylindricalcross-sectional profile shown above, the CW beam could have a variety ofshapes depending upon the requirements of a particular application. Insuch an embodiment, the shape of the applied beam would in turndetermine the shape of the nucleation site.

Yet still further, the method in accordance with one embodiment of thepresent invention could be combined with the use of seed crystals topromote crystallization. A seed crystal could be introduced into theamorphous silicon prior to superpositioned laser annealing in severalpossible ways. One method of forming a seed crystal would be by solidphase crystallization such as metal-induced crystallization orgermanium-induced crystallization. Alternatively, another form of laserrecrystallization could be employed. Use of seed crystals would beeffective to control location and orientation of the p-Si grainsultimately produced using the present invention.

Yet still further, focused micro-spots from solid state lasers could beused instead of the large spot area excimer laser beam. The use ofmultiple solid state lasers having smaller beam cross-sections woulddecrease the cost of the method and increase throughput by reducingdependence upon a single laser source.

Yet still further, delivery of the laser beams to the sample could beaccomplished with micro-optics, including MEMS scale devices. Examplesof delivery optics structures formed by micro-fabrication processes andavailable for use with the present invention include, but are notlimited to, lenses, mirrors, and beam splitters.

Yet still further, utilization of multiple and high-repetition beamscould dramatically increase the fabrication speed and efficiency in amassively parallel system. Such alternative embodiments would encompasssplitting a single beam into a large number of parts appliedindependently to the substrate, and would permit the irradiation oflarge areas of substrate more rapidly than with a single beam.

Yet still further, beam alignment and on-line probing schemes could bedeveloped and integrated in the process in order to enhance processrepeatability, stability, reliability and robustness. Micro-opticsincluding MEMS scale devices can be used for the system.

Yet still further, while the method in accordance with one embodiment ofthe present invention has been described so far primarily in connectionwith the formation of a p-Si TFT structure, the present invention is notlimited to this application. Superpositioned laser annealing ofamorphous material could be employed to create optical memory storagedevices, such as DVD disks. In such an embodiment, selectivesuperpositioned laser annealing would create regions of large-grainedpolycrystalline material representing bits of information.

Having fully described several embodiments of the present invention,many other equivalent or alternative methods and apparatuses forfabricating long crystal polycrystalline silicon according to thepresent invention will be apparent to those skilled in the art. Thesealternatives and equivalents are intended to be included within thescope of the present invention.

What is claimed is:
 1. A thin film transistor comprising a source, adrain, and a channel, the channel positioned between the source anddrain and comprising polycrystalline silicon formed from amorphoussilicon by application of a first energy beam to an amorphous siliconfilm and application of a superpositioned second energy beam to theamorphous silicon film for a second duration occurring within the firstduration.
 2. The thin film transistor of claim 1 wherein the channelcomprises an average polycrystalline grain size of greater than 10 μm.3. The thin film transistor of claim 2 wherein a grain boundary of thepolycrystalline grain is oriented at an angle of 75° or less relative toan axis between the source and drain.
 4. The thin film transistor ofclaim 1 wherein a length of the channel is formed by a singlepolycrystalline silicon grain.